The slew rate, of a load driver circuit, is commonly defined as the maximum rate of change of the voltage driving the load. Load drivers are often designed to maximize the slew rate to get fast load response time. However, with certain loads, such as inductive solenoids, it is desirable to limit the slew rate. Limiting the slew rate limits electromagnetically radiated energy that can interfere with other circuits and systems.
Prior art relies on one of several schemes to limit the slew rate. One method includes a large capacitor connected between a drive transistor's control input and the load. This type of circuit suffers from many deficiencies. One deficiency is that the capacitor needs to be large to swamp out the effect of the drive transistor's variable internal capacitance. This size restriction causes an unnecessary burden on the circuit driving the drive transistor's control input. Also, the size of the capacitor makes it impractical to fit on an integrated circuit. Therefore, this scheme is more unreliable as more parts are used.
Another prior art solution is to include a large capacitor connected between a drive transistor's control input and circuit ground. Aside from having the same deficiencies as the prior circuit, the slew rate of the voltage driving the load varies as the load changes. This happens when the load changes because of manufacturing tolerances and aging.